Loud-speaker



Jan. 22, 1929 7 1,699,903

N. w M LACHLAN LOUD SPEAKER Filed Jan. 16, 1928 2 Sheets-Sheet 2gwvenfoz /l 0/"/770/7 M/m MZM/M dbtom w i Patented den. 22, 1929.

UNITED STATES V eeann PATENT OFFICE.

NORMAN NILLIAM MCLACHLAN, OF LONDON, ENGLAND, ASSIGNOR T WIRELESS MUSICLIMITED, OF LONDON, ENGLAND.

LOUD-SPEAKER.

Application filed January 16, 1928, Serial No. 247,228, and in GreatBritain January 11, 1927.

The present invention relates to loud speakers of the type in which adiaphragm or other surface of relatively large size is in operativeconnection with a'driving element which is adapted to be set invibration electrically.

Many attempts have been made to produce a loud speaker of this type,which shall re-,

It is also difficult to design a driving element which shall give aneven response over this band of frequencies without excessive dampingwhich renders the system insensitive. The present invention takesadvantage of the fact that certain sound producing surfaces break upprogressively as the frequency applied to them is increased, that is tosay, the effective area in vibration decreases progresively withincreasing frequency. By using a driving element having a naturalfrequency higher than has been customary in the past, and by coupling toit a sound producing surface'which is of such size and of such materialand shape that it only commences to vibrate as a whole in theneighbourhood of the lowest frequency to be reproduced, a very uniformoverall response characteristic can be obtained and at the same time 40pronounced resonances are eliminated.

According to the present invention the natural frequency of the drivingelement of a loud speaker of the type set forth when detached from thesound producing surface is made greater than 2500 cycles per second.

All parts which can be regarded as vibrating efiectively with thedriving element are considered to form part thereof: Thus where acantilever reed is utilized as driving 5 member and is connected to aconical diaphragm by means of a metal rod and metal nuts, the connectingmeans are regarded as forming part of the driving means.

The sound producing surface is made of e such size and shape and of suchmaterial that it does not freely vibrate as a whole above 150 cycles persecond. The question as to whether the surface is vibrating as a wholeor not may be tested by dusting a fine powder on to the surface. If anynodes are present these will then be rendered visible by a piling up ofthe powder upon them. The presence of nodes indicates that the surfaceis not vibrating as a whole.

The natural frequency of the vibratory parts of the loud speaker, whichcomprise the driving element, the connecting means and the diaphragm, ascalculated from the stiffness of the driving element and the mass of thevibratory parts, regarded as lump loaded upon the driving element, liespreferably between 700 and 1100 cycles per second.

The diaphragm is preferably conical and supported at its periphery withonly suiiicient restraint to provide the desired terminal impedance.

Suitable means are preferably provided for minimizing the interaction ofpressure waves generated by the front and rear surfaces of the diaphragmrespectively, in order to preserve the lower tones. Such means maycomprise a box, the front end of which is sealed by the diaphragm andits flexible annular member, whilst apertures of suitable size may beformed in the rear of the box to reduce undesirable box resonance, andpossible reduction of the lower tones by interference.

The invention is illustrated in the accompanying drawings, in which Fig.1 shows diagrammatically a method of construction of a diaphragm;

Fig. 2 a schematic sectional elevation of a loud speaker in accordancewith the inventiOIl; i

Figure 3 is a sideview of a movement arranged to be mounted inthe loudspeaker casing showing a portion thereof taken along the line 33 ofFigure 4, and

Figure 4 is a plan" view of the movement.

Referring to the'drawings, the diaphragm '1, which in its'final form isconical, is made of hot press paper about 0.2 millimetres in thicknessand weighing about 0.015 grammes per square centimetre. Acircle A, 8%inches 5 radius, is first cut from the paper and two radii B and C, 60apart are drawn thereupon. One cut is then made along one of these radii(say B) and another cut D along a line parallel to and about inch fromthe other radius between the two radii and ex.- tending from the formerradius (B) to the periphery of the circle. The inch wideare made aroundthe edge of the cone extending'inwards about inch, and the tongues soformed arebent outwards --to lie in the plane of thecone base. Thesetongues are then glued on to a sheet of thin paper and when dry thecircular portion of this paper ,inside the-tongues is removed.Alternatively, instead of making the radial incisions the outer rimmay-be pressed into shape by means of a male and female jig, the outeredge of paper being previously moistened. There is now, therefore, acone of. about 18% inches base .diameter,. having therefore a projectedarea slightly less than, 1006 sq. cms. and with an apex angle of about113, bounded by a fiat annular member of thin paper, such as thatcommonly employed for charcoal sketches.

The diaphragm so formed is warmed to remove moisture and then treatedall over both sides with a suitable waterproofing yarnish, such as asolution of cellulose. After this treatment the diaphragm is preferablyleft for a week in a dry warm atmosphere. The diaphragm is then glued tothe edge of a wooden frame 2, 16 A and 17% inches wide, internally andexternally respectively, and

" about 2 inches deep, with the conical portion projecting through theframe. If desired, the edge of the thin paper of the diaphragm which isto be'glued may be left unvarnished to facilitate gluing, and when dry,varnish may be applied in the neighbourhood of the oin.

' wooden box 3, for example of inch thick a total area of about 60square inches. The

wood, whose dimensions are 17 inches square internally and 7 inchesdeep, The back of the box is provided with apertures 4 having frame carring the diaphragm is inserted within the fi'ont of the box and is heldfirmly against fillets secured around the inside of the box by suitableblocks secured to the inside of the casing in front of the frame. Thediaphragm and frame may therefore'be readily replaced if necessary. Thefront of the dia phragm in its final position is preferably about inchbehind the front edge of the box and the front of the box may be coveredwith thin silk (not shown) secured in position by means of a beading.

- V In order to strengthen the cone in the neighbourhood of its apex asmall cone 5 of the same material as the larger cone is preferably Theloud speaker casing consists of a stiff overlapped portion of thesmaller cone should be arranged at the opposite side to the overlap ofthe large cone. I

The annulus 1' of light-paper around the conical portion of thediaphragm, which annulus is of minor or negligible import as a soundradiating surface, serves to support the diaphragm to prevent thepassage of pressure waves around the periphery of the cone, to preserve.the lower tones, and to impose a terminal impedance upon the diaphragmsuch as will substantially prevent reflection and consequent resonance.Instead of paper, other materials which fulfil these conditions may beused.

The diaphragni hereinbefore described is adapted to be impulsed by areed loud speaker movement 6, through a threaded rod 7, which isattached thereto by screwed conical cups 8 and 9, as shown in Fig. 2.The movement is mounted in the rear of the box 3 and is preferablyacoustically insulated therefrom by means of a rubber ring or the like10.

As stated above, any suitable type of movement complying with therequirements of the present invention may be employed, but in order thatthe relative dimensions of the apparatus as a whole may be appreciated,one suitable movement as shown in Figures 3 and 4: will now be describedin detail.

The said movement comprises a base piece 12 of flat brass which isadapted to be attached to the-back of the box 4. A relatively large boss13, havinga central hole 1a, is provided on the rear side of the base. AU-shape'd magnet 15 is secured at its base to a plate 16 and a spindle17 secured to the plate projects therefrom away from the magnet. Thisspindle is made a good sliding fit in the hole 14 through the boss and afeatherway 18 is provided in the spindle and is engaged by a small block19 which is pressed into position by means of a screw 20 passing fromthe outside to the inside of the boss and raven-ting rotation of thespindle relative y to the boss but permitting free sliding thereof. Theend of the spindle remote from the plate is threaded to receive anadjusting knob 11, whilst a helical spring 21 surrounds a small locatingspigot 22, formed upon the base piece upon the opposite side to the boss13 andco-axial therewith, and tends to force the plate 16 carrying themagnet in a direction away from the base. The knob is provided with agroove which is engaged by a plate secured to the rear face of the boss,and this permits rotation of the knob but prevents translation thereofalthough this arrangement is not per surfaces, in order to concentratethe flux in the air gap. A coil 25, adapted to carryaudio frequencycurrents,is arranged around eachpole piece, and these coils may beconnected in series. The resistance of the coils in series may be 2000ohms. A pillar 26 is secured to and projects from the base on theopposite side to the boss, and parallel with the spindle and upon itsends, projecting at right angles to the pillar, is secured a reed 27 ofmild steel or suitable silicon steel alloy which passes just above thegap between the pole pieces. the adjusting knob permits of the movementof the magnet pole pieces relatively to the reed. The thin mild steelrod 7 is riveted into the reed at a point over the middle o'f It will bereadily understood that the air gap and projects at right angles to thereed. The mild steel rod 7 is provided with a thread upon which isscrewed a conical cup 9 into which the apex of the cone diahragm 1 seatswhen the movement is placed 1n position uponthe box (see Fig. 2).Alternatively,the conical cup may be formed integral with the rod or maybe soldered thereto. The end of the threaded rod projects through tothefront of the cone and a coni-' cal nut or cup 8 is screwed on to securethe centre of the diaphragm firmly to the rod. The angles of the conicalcup and of the coni cal nut should be approximately equal to that of thecone diaphragm.

The free length of the reel is 2 centimetres, the distance from thesupport to the threaded rod 1.25 centimetres, the width of the reed 0.64centimetres and the thickness of'the reed 0.18 centimetres. The mass ofthe connecting means (threaded rod, cup and nut) is 2.5 grammes and themass of the conical portion of the diaphragmis 23 grammes. The stiffnessof the reed, which may be of turbostalloy or steel, is found bycalculation from its dimensions to be 9 x 10 dynes per centimetre, andits natural frequency about 3500- 4000 cycles per second.

lVhere a simple cantilever reed of constant rectangular cross section isused its natural frequency may be determined by Rayleighs approximateformula:

where is the natural frequency, '15 the thickness in centimetres andZthe length in centimetres.

With the connecting means the natural frequency is approximately 2800cycles per second.

If the mass of the conical portion of the diaphragm, together with thatof the connecting means, be assumed to be lump-loaded on the reed at thepoint of attachment of the connecting means thereto, the calculatednatural frequency of the vibratory system will be found to be about 1000cycles per sec- 0nd. An allowance for the mass of the reed lowers thecalculated frequency to 950 cycles per second. The natural frequency ofthe diaphragm considered as a mass vibrating upon its paper annularmember, is below good audibility about 25 cycles per second.

Although the values given above have been found to give the bestresults, certain small 4 being between 2500 cycles and 3000 cycles persecond, said sound producing member being of such nature and dimensionsthat it does not freely vibrate as a whole above 150 cycles per second.

2. A sound reproducing device comprising a sound producing member ofrelatively large size, a vibrating reed operatively connected with saidmember, the natural frequency of said reed being greater than 2500cycles per second, said sound producing member being of such size, shapeand material that it does not freely vibrate as a whole above 150 cyclesper second.

3. A sound reproducing device comprising a sound producing member ofrelatively large size, a vibrating driving element, means operativelyconnecting said producing member with said driving element whereby saidelement is so dimensioned that its natural frequency when loaded at itspoint of attachment with said member, with a mass equal to the mass ofsaid member and the mass of said connecting means, is between 700 and1100 cycles per second, said sound producing member being of such natureand dimensions that it does not freely vibrate as a whole above 150cycles per second.

4;. A sound reproducing device comprising a conical sou'nd producingmember of relatively large size, and a'vibrating driving elementoperatively connected with said member, the natural frequency of saiddriving element being between 2500 cycles and 8000 cycles per second,said sound producing member being of such nature and dimensions that itdoes not freely vibrate as a whole above 150 cycles per second.

5. A sound reproducing device comprising a conical sound producingmember of relatively large size, a vibrating driving element operativelyconnected with said member, the natural frequency of said drivingelement bemosses which is adapted tobe set in vibration, and means forconnecting said driving element With said sound producing member,wherein 5 the natural frequency of said driving element is greater than2500 cycles per second and wherein the natural frequency of said soundproducing member, when detached from said connecting means, is less than30 cycles per 20 second.

In testimony whereof I have signed my name to this specification.

N ORMAN WILLIAM MGLACHLAN.

